1. Field of Invention
This invention relates to an implantable infusion pump. More particularly, this invention relates to an improved implantable infusion pump having a casing, a base plate, a propellant chamber, a bellow mechanism and a constriction section, wherein the bellow mechanism includes a base member and an internal volume, the bellow mechanism internal volume receiving a medicant for delivery into a patient""s body, and the constriction section communicating with the bellow mechanism internal volume and a catheter disposed within the patient""s body.
2. Description of Prior Art
Infusion pumps are known in the prior art, such as those seen in German Patent Application D 226 04 113 C2, wherein the pump is implanted into a patient who is in need of a continuous supply of with medicament over a long period of time. An example of such a patient is one who is experiencing chronic pain or spastics.
It is known from DE 196 24 215 C1 to provide an infusion pump with an electromagnetic resonant circuit, wherein one element for level measurement is mechanically connected to the base of the bellows. However, the construction of this type of infusion pump is very complicated. Moreover, as the liquid level (medicament) decreases, the measurement accuracy of such liquid also decreases thereby not providing an accurate mechanism for monitoring the medicament level present within the pump.
The problem of the invention is to create an improved implantable infusion pump permitting for easier and more accurate level determination of the medicament within the pump.
The infusion pump according to the invention is advantageous in that the measurement accuracy increases as the liquid level decreases such that the accuracy is largest where it matters most, i.e., when the liquid is exhausted to very low levels. Moreover, the construction of the infusion pump is particularly easy and compact.
The problem underlying the invention is solved by an improved implantable infusion pump including a housing, a base plate, a propellant chamber, a bellow mechanism and a constriction section, the bellow mechanism having a base member and an internal volume, the bellow mechanism internal volume receiving a medicant for delivery into a patient""s body, the constriction section communicating with the bellow mechanism internal volume and a catheter disposed within the patient""s body. The improved pump of the present invention includes a resonant circuit having an inductance and a capacitance, the resonant circuit having a resonant frequency that is determined by a distance measured between the bellow mechanism base member and the pump base plate.
According to one embodiment of the invention, the inductance is provided by a resonant circuit coil, which is preferably constructed as a flat coil. The flatness of the flat coil allows for a compact design for the implantable infusion pump. The flat coil is disposed within a depression formed on a surface of the pump base plate directed towards the bellow mechanism base member. Thus, effects of coil stray fields are minimized. The pump base plate is electromagnetically shielded. Thus, the coil field is intensified in the direction of the base member of the bellow mechanism.
The capacitance for the resonant circuit is provided by a capacitor. Thus, the capacitance is easily adaptable to the circuit frequency needs. According to one embodiment of the present invention, the propellant chamber encloses the capacitor. This arrangement is particularly advantageous when the capacitor is located in a dead space of the propellant chamber such that the volume of the propellant chamber is more effectively used. However, the capacitor can be positioned on the side of the base plate which is opposite to the propellant chamber (i.e., the top surface). This position of the capacitor is particularly safe as the medicament in the bellow mechanism internal volume is separated from the capacitor by the strong barrier provided by the base plate. Advantageously, the capacitor is positioned adjacent to the base plate.
In another embodiment of the present invention, the capacitor is provided as a parallel plate capacitor having a first plate being provided by the base plate and a second plate being provided, in parallel, to the base plate. Thus, compactness of the pump is attained and is particularly large.
In yet another embodiment, the flat coil provides the capacitance for the resonant circuit. Thus, the need for a separate capacitor can be obviated with the result of the pump being particularly compact. The compactness can be optimized by providing a strong dielectric between the capacitor plates.
According to yet another embodiment, an electrically conductive coating is disposed along a bottom surface of the bellow mechanism base member. This enhances the accuracy of the liquid level measurement. Preferably, the conductive coating is constructed of copper. In an alternate embodiment however, the electrically conductive coating is chosen from other good conductors, such as gold.